There is one mark of age that strikes me more than any of the physical ones;—­I mean the formation of Habits.  An old man who shrinks into himself falls into ways that become as positive and as much beyond the reach of outside influences as if they were governed by clock-work.  The animal functions, as the physiologists call them, in distinction from the organic, tend, in the process of deterioration to which age and neglect united gradually lead them, to assume the periodical or rhythmical type of movement.  Every man’s heart (this organ belongs, you know, to the organic system) has a regular mode of action; but I know a great many men whose brains, and all their voluntary existence flowing from their brains, have a systole and diastole as regular as that of the heart itself.  Habit is the approximation of the animal system to the organic.  It is a confession of failure in the highest function of being, which involves a perpetual self-determination, in full view of all existing circumstances.  But habit, you see, is an action in present circumstances from past motives.  It is substituting a vis a tergo for the evolution of living force.

When a man, instead of burning up three hundred pounds of carbon a year, has got down to two hundred and fifty, it is plain enough he must economize force somewhere.  Now habit is a labor-saving invention which enables a man to get along with less fuel,—­that is all; for fuel is force, you know, just as much in the page I am writing for you as in the locomotive or the legs that carry it to you.  Carbon is the same thing, whether you call it wood, or coal, or bread and cheese.  A reverend gentleman demurred to this statement,—­as if, because combustion is asserted to be the sine qua non of thought, therefore thought is alleged to be a purely chemical process.  Facts of chemistry are one thing, I told him, and facts of consciousness another.  It can be proved to him, by a very simple analysis of some of his spare elements, that every Sunday, when he does his duty faithfully, he uses up more phosphorus out of his brain and nerves than on ordinary days.  But then he had his choice whether to do his duty, or to neglect it, and save his phosphorus and other combustibles.

It follows from all this that the formation of habits ought naturally to be, as it is, the special characteristic of age.  As for the muscular powers, they pass their maximum long before the time when the true decline of life begins, if we may judge by the experience of the ring.  A man is “stale,” I think, in their language, soon after thirty,—­often, no doubt, much earlier, as gentlemen of the pugilistic profession are exceedingly apt to keep their vital fire burning with the blower up.